A conventional power supply monitoring IC will be described below with reference to FIG. 3. In FIG. 3, reference numeral 72 represents a power supply apparatus (hereafter a "battery pack") having lithium-ion cells 2 and 3 and a power supply monitoring IC 73. When the cells 2 and 3 are charged, terminals 11 and 12 are connected to a power source for charging (not shown), and, when the battery pack 72 is in use, a load (not shown) is connected to the terminals 11 and 12.
In normal use, the lithium-ion cells 2 and 3 each have a voltage from 2.3 V to 4.2 V. Accordingly, for example, the power supply monitoring IC 73, when the voltage becomes higher than 4.3 V, inhibits charging to prevent overcharging, and, when the voltage becomes lower than 2.2 V, inhibits discharging to prevent overdischarging.
Now, the portion of this conventional power supply monitoring IC 73 that detects overdischarging will be described. The portion that detects overcharging will not be described; nor is it shown in FIG. 3. Of the two lithium-ion cells 2 and 3, the cell 2 is placed on the higher potential side. The higher potential end of the cell 2 is connected to the positive terminal 11 of the battery pack 72. On the other hand, the lower potential end of the cell 3 is connected to the drain of an n-channel MOSFET (metal-oxide semiconductor field-effect transistor) 8. The source of the MOSFET 8 is connected to the negative terminal 12. The gate of the MOSFET 8 is connected to a terminal T1 of the power supply monitoring IC 73, so that the MOSFET 8 is turned on and off by the power supply monitoring IC 73.
The higher potential end of the cell 2 is connected through a protection resistor R5 to a terminal U1 of the power supply monitoring IC 73. The node between the cells 2 and 3 is connected through a protection resistor R6 to a terminal U2. The lower potential end of the cell 3 is connected to a terminal GND of the power supply monitoring IC 73.
During discharging or charging, the power supply monitoring IC 73 turns on the MOSFET 8 so that electric power is supplied from the cells 2 and 3 to an electronic appliance or the like connected to the terminals 11 and 12. On the other hand, during charging, a direct-current voltage is applied from a direct-current power source or the like to the terminals 11 and 12, and thereby the cells 2 and 3 are charged.
The protection resistors R5 and R6 have a resistance of about 1k.OMEGA. and serve to prevent infiltration of noise into the power supply monitoring IC 73 which may result in electrostatic destruction of the power supply monitoring IC 73. Moreover, the protection resistors R5 and R6 also serve to protect the cells 2 and 3 from destruction by preventing the cells 2 and 3 from being short-circuited even when the terminal U1 or U2 is short-circuited to the terminal GND.
Between the terminals U1 and U2, resistors R1 and R2 are connected in series. The voltage at the node between the resistors R1 and R2 is fed to the non-inverting input terminal (+) of a comparator 4. To the inverting input terminal (-) of the comparator 4, a voltage higher than the voltage at the terminal U2 by a reference voltage V1 is fed. The comparator 4 receives electric power via the terminal U1. Thus, the comparator 4 compares the voltage of the cell 2 with a predetermined overdischarge voltage. The overdischarge voltage is set, for example, at 2.2 V. The comparator 4 outputs a low level if the voltage of the cell 2 is lower than the overdischarge voltage, and outputs a high level if the voltage of the cell 2 is higher than the overdischarge voltage.
Between the terminals U2 and GND, resistors R3 and R4 are connected in series. The voltage at the node between the resistors R3 and R4 is fed to the non-inverting input terminal (+) of a comparator 5. The terminal GND is grounded so as to be at the ground level. To the inverting input terminal (-) of the comparator 5, a voltage higher than the ground level by a reference voltage V2 is fed. The resistances of the resistors R1 and R3 are equal, and the resistances of the resistors R2 and R4 are equal. The reference voltages V1 and V2 are equal. Thus, the voltages of the cells 2 and 3 are checked against the same overdischarge voltage.
The outputs of the comparators 4 and 5 are fed to an AND circuit 6. Thus, when the voltages of both of the cells 2 and 3 are higher than the overdischarge voltage, the AND circuit 6 outputs a high level. By contrast, when the voltage of at least one of the cells 2 and 3 is lower than the overdischarge voltage, the AND circuit 6 outputs a low level. In this way, when the voltages of both of the cells 2 and 3 are higher than the overdischarge voltage, the AND circuit 6 outputs a high level that is used as a discharge enable signal SD. The discharge enable signal SD is fed to a discharge control circuit 7.
While the discharge control circuit 7 is receiving the discharge enable signal SD, the discharge control circuit 7 applies a signal to the gate of the MOSFET 8, which is connected to the terminal T1, to turn on the MOSFET 8. By contrast, while the discharge control circuit 7 is not receiving the discharge enable signal SD, it keeps the MOSFET 8 off. As a result, the cells 2 and 3 are disconnected from the load, and thereby discharging is stopped. In this way, the cells 2 and 3 are prevented from being brought into an overdischarged state.
However, in this conventional power supply monitoring IC 73, voltage drops are caused across external impedance, such as the protection resistors R5 and R6 and wiring resistances, by the current flowing therethrough, and this causes an error in the detected voltages of the cells 2 and 3. Thus, variations in the current flowing into the power supply monitoring IC 73 and variations in external impedance degrade detection accuracy. For example, in the case of the comparator 5, which receives electric power through the resistor R6, a variation in the voltage resulting from electric power being supplied appears at the voltage division point, and such a variation appearing at the voltage division point as a result of electric power being supplied is difficult to correct. Now suppose that the power supply monitoring IC 73 monitors the overdischarge voltage with accuracy of about 50 mV, that the current flowing through the resistor R6 via the terminal U2 as the operation current of the comparator 5 is tens of microamperes, and that the resistor R6 has a resistance of 1 k.OMEGA., then a voltage drop of tens of microvolts occurs. In this way, variations in the resistances of the protection resistors, in wiring resistances, and in the operation current cause an error in detection accuracy as large as such a voltage drop, and thereby degrade detection accuracy. Furthermore, the current flowing through the resistors R1 and R2 in the upper stage flows also through the resistors R3 and R4, and this also causes an error in the voltage at the voltage division point with respect to the voltage that should be present there.
Moreover, in case the resistor R6 is disconnected from the terminal U2 by an accidental cause such as improper soldering or a mechanical shock, the resistors R1 to R4 are left connected simply in series, and therefore the comparators 4 and 5 erroneously recognize the average voltage of the cells 2 and 3 as the voltages of the cells 2 and 3, respectively. For example, if such a disconnection occurs at the terminal U2 when the voltage of one of the cells 2 and 3 equals the overdischarge voltage 2.2 V and the voltage of the other equals 3.4 V, the comparators 4 and 5 both recognize the average voltage (2.2+3.4)/2=2.8 V as the voltages of the individual cells 2 and 3 and compare this voltage with the overdischarge voltage 2.2 V. As a result, the AND circuit 6 outputs the discharge enable signal SD to continue discharging, bringing the cells into an overdischarged state.